I got 4 shaman, quite well geared though

28k hp, 3k SP unbuffed. 173 hit, 0 haste.

And i can pretty much 4 man all oldschool HC's.
Can't do ICC ones, occulus and toc.

Problem is, that i can gear up and then solo HC's. Which isn't quite usefull.

I would never go 5 shaman. Or go 4 shaman + tank or druid
or just stick to 4 shaman and let someone else tank = ezmode

Ps: no, i haven't in fact cleared all those instances with just me x 4, but i did do some with a fury warrior or a doomkin as fifth just for the daily. And i just had the shaman tank most stuff. Bosses die in 15-20 seconds of combat though.